Generally, bicycle derailleurs are provided with a base member mounted to the bicycle frame, a movable member movable axially of the sprocket assembly, the movable member carrying a chain guide, and a pair of linkage members interposed between the base member and the movable member and supporting the movable member with respect to the base member. Generally, the base member and movable member each have a pair of mounting walls opposite to each and the linkage members are inserted at both of their ends between the mounting walls to be swingable through link pins respectively.
The derailleur constructed as described above is mounted on a seat tube or a rear solid end at the bicycle frame. In either case, each member of the derailleur is exposed to the atmosphere and, during rainy weather, is splashed with rain water or muddy water from the rotating wheel.
The pivotal portions between the insertion portions at each linkage member and the mounting walls at the base member and the movable member have gaps therebetween through which rain water or muddy water may enter into the space around the link pins. Mud or debris may clog the gaps resulting in a resistance against the swinging motion of the linkage members, thus yielding a heavy lever operation for changing the bicycle speed. Also, the sliding portion may be worn out or get rusty due to the muddy water.